Open Solar Telescope Overcomes Hot-Air Turbulence

A new telescope perched 15 m above a mountaintop promises to solve many of the problems that plague telescopes designed to peer at the sun. Using a scheme that leaves the main mirror of the telescope open to ambient wind, solar researchers from Utrecht University in The Netherlands have reduced the turbulence inside a telescope that is generated by the heat of the focused image of the sun.

Fifteen meters above a mountain peak in the Canary Islands, the Dutch Open Telescope uses ambient wind to improve its image sharpness by keeping hot-air turbulence from building up between the main mirror and the image-collecting apparatus. Courtesy of Robert Hammerschlag.
Even in clear weather, turbulent atmosphere can make or break a night's work. Turbulence built up inside the telescope's tube as it is heated by the sun's beams compounds atmospheric turbulence. Researchers previously solved this problem by removing the air from the tube, but the vacuum windows required for this limited the size of the telescope and raised its cost.

The Dutch Open Telescope is an innovation in telescope design. Its 45-cm mirror is open to the elements, allowing the strong northerly trade winds off the Atlantic to wash over it. A water-cooled diaphragm above the mirror transmits only a small portion of the mirror's image to a charge-coupled device camera.

Telescope designer and astronomer Robert Hammerschlag of Utrecht University said that for the best conditions, the wind moves across the mirror at about 10 m/s. This steady, cool breeze removes turbulence between the mirror and the detection apparatus, and may cool the mirror, too. "The wind also prevents the warm bells of air caused by the ground from reaching the telescope," Hammerschlag said.

Trade winds

The wind is one reason why the scientists chose to perch their telescope atop the Roque de los Muchachos in the Canary Islands. The winds are strong but smooth because they come off the ocean. Good observation conditions can be had in winds up to 20 m/s; however, in bad weather the wind can gust to 70 m/s, and in the winter, snow and ice storms rage around the summit.

To protect the telescope from the wind and weather and provide a steady platform, the tower itself was an engineering feat. A steel spider, its eight legs are made from open tubular triangles and can handle 30 tons of snow and ice. When buffeted by winds, the tower moves parallel to the telescope, minimizing tilting and allowing the telescope to precisely track astronomical objects.

A retractable cover made of a treated polyester fabric and steel supports protects the telescope from inclement weather and remains folded during operations. Lids also protect the main mirror and the secondary optics. Even though it is open to the elements, Hammerschlag said, the astronomers have "had no problems with dirt on the mirror -- no more dirt than with a classical telescope dome." However, the mirror has a hard coating that can be cleaned.

The Dutch Open Telescope, which was funded by the Dutch Technology Foundation STW, is the test case for the new design. Researchers with the National Solar Observatory in Kitt Peak, Ariz., and Sunspot, N.M., will be watching. They are planning a similar open-air telescope with a 4-m mirror.